1. Field of the Invention
The present invention relates to a method and an apparatus for producing slurry of a mixture of liquid nitrogen and solid nitrogen, which is slush nitrogen, and a simple method for evaluating solid concentration of the same and a method for cooling using the same.
2. Description of the Related Art
Liquid nitrogen is widely used as a cooling agent. When a sherbet-like mixture of solid nitrogen and liquid nitrogen is used, its density and cooling capacity per unit mass are increased so that the mixture becomes an efficient cooling agent. However, a method for producing economically slush nitrogen comprising solid nitrogen having a homogenous and fine particle size has not been established.
Slush nitrogen has an excellent capacity of absorbing heat load compared with liquid nitrogen because a latent heat of melting of solid nitrogen is used so that slush nitrogen is effectively used for cooling an electric-power-transmission cable for high-temperature super conductivity and high-temperature super conductive apparatuses such as a magnet, a current limiting device, and a transformer, etc. Meanwhile, taking advantage of its characteristics that its density and cooling capacity per unit mass are increased, a sherbet-like mixture of solid hydrogen and liquid hydrogen attracts attention as a future fuel for an aerospace plane and its production method and apparatus are developed.
As for production methods of slush hydrogen, there are (1) a spraying method, (2) a freezing-melting method, and (3) a helium freezing method. In a spray method (1), when a low temperature vessel (cryostat) is depressurized to under 50 mmHg and liquid hydrogen is sprayed into the vessel, liquid particles are deprived of a latent heat of vaporization so that the temperature is lowered and solid hydrogen particles are generated. In a freezing-melting method (2), when a low temperature vessel containing liquid hydrogen is depressurized with a vacuum pump, hydrogen is vaporized from the liquid surface of the liquid hydrogen to generate solid hydrogen on the surface of the liquid hydrogen by being deprived of a latent heat of vaporization. The solid hydrogen is crushed mechanically to obtain slush hydrogen. In a helium freezing method (3), liquid hydrogen is filled in a low temperature vessel in which a heat exchanger is disposed; a helium gas of a temperature below 18-13 K is introduced to solidify by cooling the liquid hydrogen on the heat exchanger. The solidified hydrogen is scraped mechanically to obtain slush hydrogen (See Japanese laid-open patent publication JP06-241647).
A method for producing slush hydrogen is disclosed on Japanese laid-open patent publication JP08-285420, where solid hydrogen is generated by blowing liquid hydrogen into a depressurized low temperature vessel and liquid hydrogen is introduced into the vessel and the contents are stirred with a stirrer provided to the vessel. Furthermore, Japanese laid-open patent publication JP08-283001 discloses the following method for producing slush hydrogen. When hydrogen gas is introduced from the bottom of a low temperature vessel into which liquid helium is filled, the hydrogen is cooled to solidify while the hydrogen ascends in the liquid helium. Though the liquid helium is vaporized, if introduction of hydrogen is continued while the vaporized helium is evacuated, the vessel is almost filled with solid hydrogen. Then, liquid hydrogen is filled in the vessel to produce slush hydrogen. By this method, the internal of the vessel can be kept at the pressure greater than the atmospheric pressure so that air does not intrude in from the outward and the solid hydrogen particles in the obtained slush hydrogen are homogenously fine on account of abrupt cooling by liquid helium.
Japanese laid-open patent publication JP06-281321 discloses a method and an apparatus for producing slush hydrogen wherein liquid hydrogen is solidified on a cooled solid surface using cooling heat of liquid helium in the liquid hydrogen in a low temperature vessel (cryostat), whereby an abundant slush nitrogen is continuously produced by blowing over cooled liquid hydrogen in a low temperature vessel.
Though, in the above methods, slush nitrogen is obtained using liquid nitrogen instead of liquid hydrogen, each has the following problem. In the spray method (1), since liquid hydrogen (liquid nitrogen in case slush nitrogen is produced) is blown in the evacuated low temperature vessel, air might intrude into the vessel from the outside. In the freezing-melting method (2), air might intrude into the vessel from the outside because of depressurization of the inside of the low temperature vessel and besides; there is a drawback that particles of solid hydrogen are uneven and large. In the helium freezing method (3), particles of solid hydrogen are also uneven and large, and a particular heat exchanger is necessary.
In the case of JP08-285420, as liquid hydrogen is blown in the depressurized cooled vessel, air might intrude from the outside. Since a boiling point of liquid helium at the atmospheric pressure is 4.22 K and a melting point of solid hydrogen is 13.83 K, if a diameter of the blowing hole of a blowing nozzle immersed in liquid helium is made small in order to obtain fine particles of solid hydrogen with the method of JP08-283001, the blowing hole of the nozzle cooled below the melting point of solid hydrogen might be occluded with solid hydrogen. As a melting point of solid nitrogen is 63.17 K, which is far higher than that of solid hydrogen, if this method is applied for a production of solid nitrogen, the nozzle is occluded unless a diameter of the nozzle hole and a flow volume are large, resulting in that fine particles of solid nitrogen can not stably obtained.
Each aforementioned prior art aims at slush hydrogen production, and besides, a coolant (helium) other than object material is used. Even if the art is applied to production of slush nitrogen, an apparatus for liquefaction is necessary and a temperature has to be lower than that of nitrogen or hydrogen liquefaction when using helium that is already used as a cooling agent by recondensation thereof, whereby an apparatus becomes large and also production cost becomes high.
There has been no appropriate method for evaluating solid nitrogen concentration in slush nitrogen. If slush nitrogen flows, the concentration can be measured by a mass flow meter. As it cannot be measured unless it flows, a means for flowing is necessary. In addition, insulating device needs to be added for it is used under very low temperature, which results in high production cost. Furthermore, because nitrogen comes to be mixed in the apparatus for liquefying helium, long operation of the apparatus is difficult or an apparatus with a high performance is needed.
Meanwhile, as it is necessary to keep the temperature lower than a critical temperature of the material in order to activate a super conductive coil, a super conductive cable or others in a super-conductive state, it was conventionally cooled by immersing a body in liquid helium (b.p. 4.2 K) (for example, see JP06-77541, JP09-283321), whereas as research and development of super conductive material is advanced, a material having a high critical temperature has been found and utilized, a cooling temperature has become high. On account of emergence of high temperature super conductive material, liquid nitrogen (b.p. 77 K) can be used instead of costly liquid helium so that it has become extremely advantageous to put into practical use.
When liquid nitrogen is used to cool a super conductive apparatus by immersing in liquid nitrogen, a variety of ideas are made against bubble formation in liquid nitrogen by heat generation due to AC loss or heat intrusion from the outside, as it deteriorates insulation properties. For example, liquid nitrogen is cooled under the boiling point of liquid nitrogen to use, the boiling point is raised by pressurizing or both methods are joined. However, a temperature that cools liquid nitrogen of a melting point of 63 K without solidifying is limited to 65 K at best. An upper limit just before boiling is about 75 K. That means a temperature range capable of cooling by a sensible heat of liquid nitrogen is 10 degrees variation. Since a specific heat of liquid nitrogen is 2 kJ/kg, a heat capacity that a sensible heat of liquid nitrogen has per unit mass of liquid nitrogen is merely 20 kJ/kg. Further, as a matter of fact, it is usual that a performance of a cooled super conductor is stably higher at the temperature in the vicinity of a freezing point than in the vicinity of a boiling point of liquid nitrogen.
More specifically, as a temperature range capable of cooling with liquid nitrogen as a liquid state utilizing a sensible heat thereof is narrow and a heat capacity is small, a vast amount of liquid nitrogen is necessary for cooling (eliminating heat) so that a super conductive apparatus becomes large in size. If a cooling temperature rises to about a boiling point with this method, the performance of a super conductive device is limited to that temperature.